Bimolecular reactions solution phase

In studies where cholesteric phase order appears to influence the course of bimolecular reactions, solute diffusion and collisional orientations appear to be affected by solvent anisotropy. An example is the stereoselective photodimerization of 1,3-dlmethylthymine(24). While all four possible cis-fused cyclobutane photodimers are produced in isotropic solutions and disordered glasses, the cis-syn dimer is formed almost exclusively in liquid-crystalline media. Furthermore, selectivity of reaction products in the mesophases is greatly decreased upon addition of an isotropic diluent which disturbs local solvent order (e.g., dioxane or DMSO). Since all of the... 

It was pointed out that a bimolecular reaction can be accelerated by a catalyst just from a concentration effect. As an illustrative calculation, assume that A and B react in the gas phase with 1 1 stoichiometry and according to a bimolecular rate law, with the second-order rate constant k equal to 10 1 mol" see" at 0°C. Now, assuming that an equimolar mixture of the gases is condensed to a liquid film on a catalyst surface and the rate constant in the condensed liquid solution is taken to be the same as for the gas phase reaction, calculate the ratio of half times for reaction in the gas phase and on the catalyst surface at 0°C. Assume further that the density of the liquid phase is 1000 times that of the gas phase. 

These problems can be somewhat overcome by a study of reactions in solution where much greater densities are possible than in the gas phase and fast bimolecular reaction are diffusion limited [1,28,29]. However, since coordinatively unsaturated metal carbonyls have shown a great affinity for coordinating solvent we felt that the appropriate place to begin a study of the spectroscopy and kinetics of these species would be in a phase where there is no solvent the gas phase. In the gas phase, the observed spectrum is expected to be that of the "naked" coordinatively unsaturated species and reactions of these species with added ligands are addition reactions rather than displacement reactions. However, since many of the saturated metal carbonyls have limited vapor pressures, the gas phase places additional constraints on the sensitivity of the transient spectroscopy apparatus. 

Rapid bimolecular reactions are limited by diffusion of reactants in the liquid and solid phases. Diffusion occurs in polymers much more slowly than in liquids. Hence, such rapid reactions as recombination of free radicals occurs in polymers with rate constants of a few order of magnitude more slowly than in solution. For example, the reaction of sterically hindered phenoxyl with the peroxyl radical... 

The first step is a bimolecular reaction leading to the formation of a hydrogen bond the second step is the breaking of the hydrogen bond such that the protonated species H B+ is formed the third step is the dissociation reaction to form the products. In aqueous solutions, the bimolecular reaction proceeds much faster than would be predicted from gas phase kinetic studies, and this underscores the complexity of proton transfer in solvents with extensive hydrogen-bonding networks capable of creating parallel pathways for the first step. In their au-... 

The effect of the medium on the rates and routes of liquid-phase oxidation reactions was investigated. The rate constants for chain propagation and termination upon dilution of methyl ethyl ketone with a nonpolar solvent—benzene— were shown to be consistent with the Kirkwood equation relating the constants for bimolecular reactions with the dielectric constant of the medium. The effect of solvents capable of forming hydrogen bonds with peroxy radicals appears to be more complicated. The rate constants for chain propagation and termination in aqueous methyl ethyl ketone solutions appear to be lower because of the lower reactivity of solvated R02. .. HOH radicals than of free RO radicals. The routes of oxidation reactions are a function of the competition between two R02 reaction routes. In the presence of water the reaction selectivity markedly increases, and acetic acid becomes the only oxidation product. 

As was demonstrated by Kikuchi and Brush [88], using the Ising model as an example, an increase of mo in the expansion in the form secures the monotonic approach of the calculated critical parameters to exact results, except for the critical exponents which cannot be reproduced by algebraic expressions. It is important to note here that the superposition approximation permits exact (or asymptotically exact) solutions to be obtained for models revealing the critical point but not the phase transition. This should be kept in mind when interpreting the results of the bimolecular reaction kinetics obtained using approximate methods. 

Tetramethylethylene. Very recently, product studies on the 03 + tetramethylethylene (TME) reaction have been made by the authors group in attempts to probe various reaction channels operative for the dimethyl-substituted Criegee intermediate (CH3)2C—OO under atmospheric conditions [130]. Among the potentially important reaction channels suggested by previous theoretical studies [131] and experimental results in the gas- and solution-phases [122,132] are the bimolecular reaction with aldehydes (reaction (46c)) and the following unimolecular processes ... 

The well-known Maxwell-Boltzmann distribution for the velocity or momentum associated with the translational motion of a molecule is valid not only for free molecules but also for interacting molecules in a liquid phase (see Appendix A.2.1). The average kinetic energy of a molecule at temperature T is, accordingly, (3/2)ksT. For the molecules to react in a bimolecular reaction they should be brought into contact with each other. This happens by diffusion when the reactants are dispersed in a solution, which is a quite different process from the one in the gas phase. For fast reactions, the diffusion rate of reactant molecules may even be the limiting factor in the rate of reaction. 

From these experiments on molecular clusters, a new aspect of ion-molecule reactions can be studied as a function of the cluster size and compared with results on the gas phase and in the liquid phase. The role of the solvent in bimolecular reactions has been evidenced and the catalytic effect depends on the number of molecules (which represents a new result compared with experiments in solution) and the nature of the solvent. These systems appear to be very good systems to study the chemistry under microsolvation. 

Diphenylsilene (19a), produced by photolysis of 1,1-diphenyl- or 1,1,2-triphenylsilacyclobutane (17a and 18, respectively equation 11), has been particularly well studied, and absolute rate constants have been reported for a wide variety of silene trapping reactions in various solvents at room temperature (see Table 3)40-46. Not all of these have been accompanied by product studies, unfortunately. A number of other transient silenes have been characterized as well with solution-phase kinetic data for a range of bimolecular silene trapping reactions, though much less extensively than 19a. These include the cyclic l,3,5-(l-sila)hexatriene derivatives 21a-c (formed by photolysis... 

In contrast, when An 0 one finds considerable disagreement in the literature concerning relative gas and liquid equilibrium (and rate) constants, even in the absence of solvation effects. Depending on the particular theoretical treatment, bimolecular reaction rate constants have been estimated to be somewhere between 2 and 100 times faster in solution than in the gas phase (6). However, the very limited experimental data available indicate that there is little, if any, systematic difference between bimolecular rate constants in the two phases (6d). 

Up to the present time there are three cases where a thermal, bimolecular reaction appears to be the same in gas phase and solution. The thermal decomposition of chlorine monoxide is complicated but the time required to decompose from twenty per cent to sixty per cent at 65° is the same in carbon tetrachloride solution as in the gas phase.3... 

As for bimolecular reactions, collision theory can also be used to describe the kinetics of interfacial reactions between a solid surface and solutes in the liquid phase. Astumian and Schelly have described the theory for the kinetics of interfacial reactions in detaiL The complete rate expression, derived by Astumian and Schelly, for solutes reacting with suspended solid spherical particles is given by Eq. (1)... 

Transition state theory was also developed as a means of rationalizing rate constants for gas phase reactions and their temperature dependence. It is most directly applied to bimolecular reactions and is based on three fundamental postulates for reactions in solution ... 

Solvation takes place within 100-1000 fs. Reactions in the solution phase take place in a cage of solvent molecules. Bimolecular reactions in the solvent cage take place within several hundred femtoseconds, whereas colhsions in the gas phase take place in the order of picoseconds. In the solvent cage, molecules A and B collide with each other, and a successful collision leads to reaction to give product P. Excess energy from P is transferred to solvent molecules by the subsequent collision with solvent molecules. Therefore, one of the most important roles of the solvent is removal of heat generated in the reaction. In the solution phase, the rate of a chemical reaction is determined by the activation energy. This is mostly... 

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